Friday, February 12, 2010

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"According to Hogan, the holographic principle radically changes our picture of space-time. Theoretical physicists have long believed that quantum effects will cause space-time to convulse wildly on the tiniest scales. At this magnification, the fabric of space-time becomes grainy and is ultimately made of tiny units rather like pixels, but a hundred billion billion times smaller than a proton. This distance is known as the Planck length, a mere 10-35metres. The Planck length is far beyond the reach of any conceivable experiment, so nobody dared dream that the graininess of space-time might be discernable.

That is, not until Hogan realised that the holographic principle changes everything. If space-time is a grainy hologram, then you can think of the universe as a sphere whose outer surface is papered in Planck length-sized squares, each containing one bit of information. The holographic principle says that the amount of information papering the outside must match the number of bits contained inside the volume of the universe.

Since the volume of the spherical universe is much bigger than its outer surface, how could this be true? Hogan realised that in order to have the same number of bits inside the universe as on the boundary, the world inside must be made up of grains bigger than the Planck length. "Or, to put it another way, a holographic universe is blurry," says Hogan.

This is good news for anyone trying to probe the smallest unit of space-time. "Contrary to all expectations, it brings its microscopic quantum structure within reach of current experiments," says Hogan. So while the Planck length is too small for experiments to detect, the holographic "projection" of that graininess could be much, much larger, at around 10-16 metres. "If you lived inside a hologram, you could tell by measuring the blurring," he says.

When Hogan first realised this, he wondered if any experiment might be able to detect the holographic blurriness of space-time. That's where GEO600 comes in."